When it is desired to form a thin film of silicon single crystal (which film will be sometimes referred to merely as the thin film, hereinafter) on a silicon single crystal substrate (which will be sometimes referred to merely as the substrate, hereinafter), a pre-treatment step and a vapor phase growth step have conventionally been sequentially carried out with use of such an apparatus as shown in FIG. 5.
(1) Pre-Treatment Step (of removing a silicon oxide
film on a main surface of a substrate):
In such an apparatus as shown in FIG. 5, a silicon single crystal substrate 2 is placed on a susceptor 8 within a reaction vessel 11, and a nitrogen gas is supplied into the reaction vessel 11 to purge air within the vessel. Next, after the nitrogen gas has been purged with a hydrogen gas, the substrate 2 is heated by a radiation heater 3 provided on upper and lower sides of a vessel wall 1 to such a temperature suitable for the pre-treatment as, e.g., about 1200.degree. C. for a predetermined time, e.g., for about one minute while the hydrogen gas is continuously supplied into the vessel. In the pre-treatment step, the temperature of the substrate is set and kept at a level higher than the temperature in the vapor phase growth step, whereas an internal pressure of the vessel is kept at, e.g., atmospheric pressure (which will be expressed in terms of absolute pressure in this specification, hereinafter). PA1 After the pre-treatment step, the substrate 2 is heated by the radiation heater 3 to and kept at a temperature suitable for vapor phase growth. Under this condition, a reactant gas 4 consisted of a carrier gas and reaction source material is made to flow along the main surface of the substrate 2. When the substrate 2 has a temperature suitable for a thin film growth, e.g., of 800 to 1200.degree. C., a chemical reaction of the reactant gas 4 causes a thin film to grow on the surface of the substrate 2.
Since a native oxide film, that is, silicon oxide film formed on the substrate 2 due to oxygen in the air (or a silicon oxide film intentionally formed) is removed by hydrogen reduction in a high-temperature during the pre-treatment step, there can be formed a silicon single crystal thin film which has a good crystallization in the vapor phase growth step.
In the pre-treatment step, since the silicon single crystal substrate is heat-treated at a high temperature of 1200.degree. C. in a hydrogen atmosphere, so that a quantity of oxygen atoms in the vicinity of the surface of the substrate is reduced by outdiffusion during the heat treatment. Thus, when a silicon single crystal thin film is grown and then subjected to a series of heat treatments, a denuded zone (or DZ layer) having a remarkably less number of defects and having a width of about 10 .mu.m is formed in the vicinity of the surface of the substrate.
Interstitial oxygens contained within the single crystal grown by a Czochralski method, when subjected to a series of heat treatments, form oxygen precipitates. Since the oxygen precipitates have an ability to getter such heavy metals as Cu or Ni atoms which lead to generation of a leakage current, a certain amount of interstitial oxygen is intentionally included in the single crystal to prevent the heavy metal pollution.
In a CZ substrate containing interstitial oxygens, however, if the amount of oxygens are not decreased sufficiently in the vicinity of the surface of the CZ substrate, then crystal defects will take place even in a silicon single crystal thin film formed directly above the CZ substrate. For this reason, it is important to heat-treat the silicon single crystal substrate at a high temperature before the silicon single crystal thin film (on which an integrated circuit is to be formed) is to be grown by the vapor phase growth step, to form the DZ layer.
(2) Vapor Phase Growth Step (forming a silicon single crystal thin film):
In this case, radiation emitted from the substrate 2 is caught by a radiation thermometer 5, which in turn sends an intensity of the caught radiation to a computer 6 to convert it to a temperature. The computer 6 then sends the converted temperature to a temperature controller 7 to supply a necessary power to the radiation heater 3.
When a substrate which has an impurity concentration not less than 1.times.10.sup.18 atoms/cm.sup.3 is employed, dopants tend to be liberated from the substrate through their outdiffusion or etching and to scatter within the reaction vessel in the pre-treatment step. For this reason, liberated dopants tend to easily mix into the silicon single crystal thin film during the vapor phase growth. As a result, a dopant concentration at an interface between the thin film and substrate varies actually gently with a very dull gradient, which disadvantageously results in a large transition width, though it is desirable to abruptly change the dopant concentration. A transition width means a width necessary for the transition from the dopant concentration of a substrate to the dopant concentration of a thin film.
As means for solving these problems, it is considered to set the temperatures of the pre-treatment step and vapor phase growth step at levels (e.g., 1000.degree. C. or less) somewhat lower than the conventional levels. However, this involves a new problem that, when the pre-treatment is not carried out for a considerably long time, the DZ layer will not be substantially formed or not formed at all in the vicinity of the surface of the substrate.
The present invention has been made in view of the above problems in the prior art, and a first object thereof is to provide a silicon single crystal and a method for fabricating a silicon single crystal thin film, which can solve the above problems in the prior art and which can form a DZ layer having substantially the same quality as when the aforementioned pre-treatment is carried out.
A second object of the present invention is to fabricate a silicon single crystal thin film which is doped with a low concentration of impurity, which is formed by a vapor phase growth process on a silicon single crystal doped with a high concentration of impurity, and in which a dopant concentration at an interface between the silicon single crystal and thin film abruptly changes.